1. Field of the Invention
The present invention relates to an electroluminescent display, and more particularly, to an organic electroluminescent display device having a buffer layer between a cathode and an organic layer.
2. Discussion of the Related Art
Lately, demands for flat display panels occupying less space increase as a display tends to increase in size. Rapidly developed is a technology of an organic electroluminescent (hereinafter abbreviated EL) device called an organic light-emitting diode (OELD) as one of such flat display panel devices.
An organic EL display is a device emitting light in a following manner. First of all, electric charges are injected into an organic layer formed between a hole injection electrode (anode) and an electron injection electrode (cathode). Electrons are then coupled with holes to form pairs, respectively, and then disappear so as to emit light.
It is advantageous that such a device can be driven by a predetermined voltage (ex. 5˜10V) lower than that of a plasma display panel (PDP) or an inorganic EL display. Hence, many efforts are made to research and develop the organic EL device.
The organic EL device has excellent characteristics of wide viewing angle, high-speed response, high contrast, and the like, thereby being applicable to a pixel of a graphic display, a pixel of a TV video display, or a pixel of a surface light source. The device can be formed on a flexible transparent substrate such as a plastic substrate. The device can be fabricated extremely thin and light as well as provide excellent color tones. Hence, the organic EL device is suitable for the next generation flat panel display (FPD).
Moreover, the organic EL device enables to realize three colors of green, blue, and red, requires no backlight of a well-known liquid crystal display (LCD) so as to consume less power, and has the excellent color sense so as to become a matter of concern to realize the next generation full color display device.
FIG. 1 illustrates a cross-sectional view of a general organic EL display device. A process of fabricating a conventional organic EL display device is explained in order as follows.
First of all, an anode material is coated on a transparent substrate. In this case, ITO (indium tin oxide) is mainly used as the anode material.
A hole injection layer (HIL) is formed on the anode material. CuPc (copper phthalocyanine) is mainly used as a material of the hole injection layer and formed about 10˜30 nm thick.
Subsequently, a hole transport layer (HTL) is formed. TPD{N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine} or NPD{4,4′-bis[N-(1-naphthy)-N-phenyl-amino]biphenyl} is used as a material of the hole transport layer and formed about 30˜60 nm thick.
An organic light-emitting layer is formed on the hole transport layer. In this case, a dopant can be added to the organic light-emitting layer. For instance, Alq3{tris(8-hydroxy-quinolate)aluminum} is used as a basic material of the organic light-emitting layer, and deposited about 30˜60 nm thick. In this case, Coumarin 6 or Quinacridone (Qd) is widely used as a green dopant in general. Moreover, DCM, DCJT, DCJTB, or the like is mainly used as a red dopant for red luminescence.
And, an electron transport layer (ETL) and an electron injection layer (EIL) are successively deposited on the organic light-emitting layer.
Occasionally, in case of green luminescence, electron injection/transport layers are not formed additionally since Alq3 used as the material of the organic light-emitting layer has the excellent electron transport capacity.
Subsequently, LiF or Li2O about 5 Å thick or alkaline metal or alkaline earth metal such as Li, Ca, Mg, Sm, or the like about 200 Å thick as an electron injection layer is coated thin thereon, thereby improving the electron injection.
Thereafter, Al is coated about 1,000 Å thick on the electron injection layer as a cathode. Unfortunately, the material of the electron injection layer such as LiF, Li2O, or the like used for facilitating the electron injection into the organic light-emitting layer weakens the adhesion between the electron transport layer and cathode.